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SubjectRe: a major regression in recent kernels? - was: Re: Null pointer OOPS in sync_inodes_sb+0xa9/0x104
Hmm. Adding some more people to the cc - in particular Jens. I'm not
sure that he'd be on the fsdevel list (although maybe he is).

The whole "backing_dev_info" has been a total disaster. The thing is
crap. It violates all the normal kernel memory management rules ("Thou
shalt use reference counts and free only when it goes to zero") and
the whole thing has been a constant source of "oh, that driver didn't
set it, but we changed all the code to require it to be correct".

And the reason we set it to NULL when the device goes away is exactly
that it's not ref-counted correctly, so we really _have_ to set it to
NULL, because it's not going to be around.

(And the reverse of that is why all kernel data structures should use
refcounts, and not some external lifetime notion)

Gaah. The caller has already done the check for a NULL s_bdi:

/*
* This should be safe, as we require bdi backing to actually
* write out data in the first place
*/
if (!sb->s_bdi || sb->s_bdi == &noop_backing_dev_info)
return 0;

before it calls into "sync_inodes_sb(sb);", but since that stupid
disconnect event can happen at any time, that doesn't protect
anything. The s_bdi field clearly just becomes NULL after the check.

Jens? This was introduced in 592b09a42fc3 ("backing-dev: ensure that a
removed bdi no longer has super_block referencing it") over a year
ago, but the _real_ problem goes back all the way to commit
32a88aa1b6df which introduced that broken "s_bdi" without refcounts to
begin with.

Guys, any idea on how to fix this? The hacky way might be to introduce
a dummy backing_dev_info and instead of setting s_bdi to NULL, we set
it to the dummy one that doesn't do anything. It's still hacky as
hell, though. The real problem really is that having pointers to
structures without refcounts is just _always_ wrong.

See Documentation/CodingStyle: "Chapter 11: Data structures":

"Remember: if another thread can find your data structure, and you don't
have a reference count on it, you almost certainly have a bug."

wiser words have seldom been spoken.

Linus

On Wed, Mar 2, 2011 at 2:52 AM, Anton Altaparmakov <aia21@cam.ac.uk> wrote:
> Hi Linus,
>
> I am not sure who the "responsible" person for this is but a problem is emerging on recent kernels where device hot plug and/or suspend/resume (which I guess effectively performas hot plug/unplug as part of its operation) causes kernel crashes with NULL pointer dereference.  It has now been reported at least by three different people on LKML/fs-devel and also one of Tuxera's customers is seeing it too on their embedded hardware.
>
> The problem appears to be that on device unplug, the s_bdi field in the struct super block is set to NULL.  But at that point a file system is still mounted and fully operational!  So any file system / VFS code that goes through s_bdi causes a NULL pointer dereference.
>
> Not sure what the correct solution is but this is definitely a functional regression given I now am aware of four independent reports in recent kernels but never before.
>
> Here are links to the reports in the archives:
>
>        https://lkml.org/lkml/2010/12/9/436
>
>        http://www.gossamer-threads.com/lists/linux/kernel/1345992
>
> And below is the third report which appears to be the same problem as the above two, i.e. the super block's s_bdi is NULL, so dereferencing it causes a NULL pointer dereference and sync_inodes_sb() contains a call to bdi_queue_work() which is likely inlined automatically by gcc and the crash is thus when bdi is first dereferenced in bdi_queue_work()...
>
> Not sure what the solution is.  I don't think it is a matter of checking against s_bdi being NULL because it is used in a lot of places so it will be hard to track them all down.  More likely s_bdi should not be set to NULL until all super_block structures referencing it go away and the disappeared device driver code should be adapted to match but I may be wrong...
>
> Just thought you should be made aware as this problem appears to be ignored / stay under the radar of people who actually have a hope of fixing it properly so far...
>
> Best regards,
>
>        Anton
>
> On 2 Mar 2011, at 03:44, George Spelvin wrote:
>> Just after resuming from a suspend to RAM, 2.6.38-rc6 plus Eric Dumazet's
>> [PATCH] net: Add default_advmss() methods to blackhole dst_ops
>>
>> Core 2 duo, 32-bit kernel, 2 GiB RAM.
>> I suspend to RAM every few days; this is not the first
>> time since booting.
>>
>> The display hadn'd flipped to X yet, so I managed to capture the screen
>> (manually transcribed):
>>
>> BUG: unable to handle kernel NULL pointer dereference at 00000030
>> IP: [<c10a3a21>] sync_inodes_sb+0xa9/0x104
>> *pde = 00000000
>> Oops: 0000 [#1] SMP
>> last sysfs file: /sys/devices/pci0000:00/0000:00:1d.1/usb3/3-1/3-1.3/3-1.3:1.0/input11/name
>> Modules linked in: btusb sco rfcomm l2cap crc16 bluetooth ctr twofish_generic twofish_i586 twofish_common serpent xcbc sha256_generic b43 mac80211 cfg80211
>>
>> Pid: 23367, comm: mount Not tainted 2.6.38-rc6 #228 Dell Inc. MXC061                          /0MG532
>> EIP: 0060:[<c10a3a21>] EFLAGS: 0010246 CPU: 1
>> EIP is at sync_inodes_sb+0xa9/0x104
>> EAX: c003fd5c EBX: f58f606c ECX: 00000000 EDX: 00000909
>> ESI: d4cc1424 EDI: c003fd0c EBP: 00000000 ESP: f56ffee8
>> DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
>> Process mount (pid: 23367, ti=f56fe000 task=c28472a0 task.ti = f56fe000)
>> Stack:
>> 7fffffff f58f6000 00000001 00000000 f56ffef8 f56ffef8 f56fff04 00000000
>> 00000303 f56fff0c f56fff0c f58f6000 00000001 00000000 f4a57000 c10a6c72
>> f58f6000 00000020 00000000 c108e442 00000000 00000000 00000020 00000000
>> Call Trace:
>> [<c10a6c72>] ? _sync_filesystem+0x3a/0x69
>> [<c108e442>] ? do_remount_sb+0x59/0xce
>> [<c10a067b>] ? do_mount+0x214/0x6a8
>> [<c10a0b83>] ? sys_mount+0x74/0xa9
>> [<c1002750>] ? sysenter_do_call+0x12/0x26
>> Code: b9 3a c1 e8 f4 6d f8 ff b8 80 f7 51 c1 31 f6 e8 f1 09 26 00 8b 7b 6c 83 c3 6c 83 ef 50 eb 44 f6 47 30 38 75 38 8b af c0 00 00 00 <83> 7d 30 00 74 2c 89 f8 e8 1a 85 ff ff fe 05 80 f7 51 c1 89 f0
>> EIP: [<c10a3a21>] sync_inodes_sb+0xa9/0x104 SS:ESP 0068:f56ffee8
>> CR2: 0000000000000030
>>
>> I can't make much sense out of it, but maybe someone can.  It's a simple
>> laptop with one rotating hard drive.  Distro is Debian/unstable, kernel
>> is hand-compiled from Linus' git.
>
> --
> Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
> Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
> Linux NTFS maintainer, http://www.linux-ntfs.org/
>
>
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